Call Admission Control

Call Admission Control (CAC) is a
deterministic and informed decision that is made before a network session is
established and is based on whether the required network resources are
available to provide suitable quality of service (QoS) for the new session.

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Overview of Call
Admission Control for IP Sessions

In Service Provider
WiFi deployments, the mobility of subscribers contributes to more dynamics when
compared to wireline broadband deployments. This mobility translates into a
high-average session churn rate and potential session count peaks during busy
periods. For the Intelligent Wireless Access Gateway (iWAG) to function
effectively under such conditions, the Call Admission Control (CAC) feature
must be configured to help control IPoE session establishment.

Note

In this document,
a Forwarding Processor (FP) corresponds to the Embedded Services Processor
(ESP) component on the ASR 1000 Series Aggregation Services Routers.

The following CAC
features are implemented from Cisco IOS XE Release 3.11S onwards:

A threshold is
set for the maximum number of authenticated subscribers on the router,
including the Point-to-Point Protocol over Ethernet (PPPoE) and IP over
Ethernet (IPoE) sessions, but not the walk-by sessions.

To prevent
low-memory conditions, the router stops accepting a session when the committed
memory from a route processor (RP) or a FP is above a specific percentage
value. This also enables you to upgrade the hardware and memory with ease.

To prevent
low-memory conditions from occurring, the router stops accepting a session when
the used memory from the Cisco Quatum Flow Processor (QFP) is above a specific
percentage value.

To prevent high
Buffer, Queue, and Scheduler (BQS) active queue usage, the router stops
accepting new sessions when the BQS active queue usage is above a specific
percentage value.

To prevent high
CPU load conditions, the router stops accepting new sessions when the per-core
CPU utilization per active FP or RP is above a specific percentage value.

Note

After the
configured session limit under CAC is attained, single-stack sessions cannot be
converted into dual-stack sessions.

The following IP
session initiators are supported by the Call Admission Control (CAC) on a data
plane:

Unclassified MAC
address IPv4 initiator

Unclassified
IPv4 initiator

Dynamic Host
Configuration Protocol v4 (DHCPv4) initiator

Unclassified MAC
address IPv6 initiator types:

IPv6 Neighbor Discovery
(ND) packets (RS/NS/NA) that are dropped by the control plane (i.e. RP)

IPv6 data packets that
are dropped at the data plane (i.e. FP)

Platform System
Resource Monitor

A platform system
resource monitor process is created when any of the CAC features listed in the
following table are configured on the Cisco ASR 1000 Aggregated Services Router
platform using the corresponding command:

Table 1 CAC Features and
their Platform System Resource Monitor Commands

CAC Feature

Command

CPU

platform subscriber cac
cpu

MEM

platform subscriber cac
mem

BQS

platform subscriber cac bqs

The platform
resource monitor process is disabled when all of the above CAC features are
disabled on the platform.

The platform
resource monitor process periodically monitors system resources such as CPU,
memory, and Buffer, Queue, and Scheduler (BQS). You can set the periodic
monitoring interval using the following command:

platform subscriber cac timerinterval

The timer value specifies the frequency at which the CAC process
is polled to check platform resources. The valid interval value is in the range
of 1 to 10 seconds with a granularity of 1 second. The default value is 5
seconds.

Note

Committed Memory
is used for monitoring subscriber CAC memory resource utilization. It provides
an estimate of the percentage of RAM that is needed to ensure a 99.99 percent
guarantee that there is never an Out Of Memory (OOM) condition during
worst-case conditions. Normally, the kernel will over commit memory by sharing
the memory between several processes. A problem with the actual memory
availability can be seen only when the memory is being used. While the
Committed Memory is a good indicator of the memory status, the value is less
dynamic. That is, if you use the
platform subscriber cac
mem command as part of CAC, a session may stay rejected for a
long period of time if it reaches the memory limit. While this may be a good
indicator of the decreasing memory resources, it may not be useful in
production environments. We recommend that if you are using the
platform subscriber cac
mem command, ensure that the values are at least at the 95
percent threshold to enable additional troubleshooting.

CAC is applicable to
any initiator for IP sessions such as unclassified MAC or DHCP-initiated
sessions. To enable CAC, the
call admission
new-model command must always be configured.

The
call admission
limit command specifies the total session charge the system will
accept before it starts rejecting incoming calls. In the above example, the
cpu-limit of 90 means incoming calls will be dropped when the measured 5-second
CPU utilization is 90% or higher.

Depending on which
condition occurs first, cpu-limit or call admission limit (charge limit), the
IP sessions will be rejected. The
call admission
ip command specifies the charge for a single IP session. In the
example given above, the charge for a single session is 10 is and lifetime of
the fixed charge (2s) is 1.

In the example
above, CAC will accept 79 CPS based on the following calculation:

No new IP session
will be allowed in the system if the CPU on the active RP is greater than 95
percent; also, the FSOL packet is immediately dropped at the data plane layer.

Displaying the
Call Admission Control Feature Statistics in Detail

The
show call admission
statistics detailed
command displays the CAC statistics in detail. It also
provides packet drop statistics for each CAC type.

Note

The
show call admission statistics detailed
command only displays the details of platform
resources, which are already configured. For example, the detail for the MEM_RP
field is displayed in the output, only if it has been configured. Otherwise, it
is not displayed in the output.

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Feature
Information for Call Admission Control

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Table 2 Feature Information
for Call Admission Control

Feature Name

Releases

Feature Information

Call Admission Control

Cisco IOS XE Release 3.11

Call Admission Control (CAC) is a deterministic and informed
decision that is made before a network session is established and is based on
whether the required network resources are available to provide suitable
quality of service (QoS) for the new session.